Method and system for fracturing a pattern using charged particle beam lithography with multiple exposure passes having different dosages

ABSTRACT

In the field of semiconductor production using charged particle beam lithography, a method and system for fracturing or mask data preparation or proximity effect correction is disclosed, wherein base dosages for a plurality of exposure passes are different from each other. Methods for manufacturing a reticle and manufacturing an integrated circuit are also disclosed, wherein a plurality of charged particle beam exposure passes are used, with base dosage levels being different for different exposure passes.

RELATED APPLICATIONS

This application: 1) is related to Zable, U.S. patent application Ser.No. ______, entitled “Method and System For Fracturing a Pattern UsingCharged Particle Beam Lithography With Multiple Exposure Passes WhichExpose Different Surface Area” (Attorney Docket No. D2SiP026b) filed oneven date herewith; and 2) is related to Zable, U.S. patent applicationSer. No. ______, entitled “Method and System For Fracturing a PatternUsing Charged Particle Beam Lithography With Multiple Exposure Passes”(Attorney Docket No. D2SiP026c) filed on even date herewith; both ofwhich are hereby incorporated by reference for all purposes.

BACKGROUND OF THE DISCLOSURE

The present disclosure is related to lithography, and more particularlyto methods for using a charged particle beam writer to manufacture asurface which may be a reticle, a wafer, or any other surface.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, optical lithography may be used to fabricate thesemiconductor devices. Optical lithography is a printing process inwhich a lithographic mask or photomask manufactured from a reticle isused to transfer patterns to a substrate such as a semiconductor orsilicon wafer to create the integrated circuit. Other substrates couldinclude flat panel displays or even other reticles. Also, extremeultraviolet (EUV) or X-ray lithography are considered types of opticallithography. The reticle or multiple reticles may contain a circuitpattern corresponding to an individual layer of the integrated circuit,and this pattern can be imaged onto a certain area on the substrate thathas been coated with a layer of radiation-sensitive material known asphotoresist or resist. Once the patterned layer is transferred the layermay undergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Eventually, a combination of multiplesof devices or integrated circuits will be present on the substrate.These integrated circuits may then be separated from one another bydicing or sawing and then may be mounted into individual packages. Inthe more general case, the patterns on the substrate may be used todefine artifacts such as display pixels, holograms, or magneticrecording heads.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, maskless direct write may also be used to fabricatethe semiconductor devices. Maskless direct write is a printing processin which charged particle beam lithography is used to transfer patternsto a substrate such as a semiconductor or silicon wafer to create theintegrated circuit. Other substrates could include flat panel displays,imprint masks for nano-imprinting, or even reticles. Desired patterns ofa layer are written directly on the surface, which in this case is alsothe substrate. Once the patterned layer is transferred the layer mayundergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Some of the layers may be written usingoptical lithography while others may be written using maskless directwrite to fabricate the same substrate. Eventually, a combination ofmultiples of devices or integrated circuits will be present on thesubstrate. These integrated circuits are then separated from one anotherby dicing or sawing and then mounted into individual packages. In themore general case, the patterns on the surface may be used to defineartifacts such as display pixels, holograms, or magnetic recordingheads.

Two common types of charged particle beam lithography are variableshaped beam (VSB) and character projection (CP). These are bothsub-categories of shaped beam charged particle beam lithography, inwhich a precise electron beam is shaped and steered so as to expose aresist-coated surface, such as the surface of a wafer or the surface ofa reticle. In VSB, these shapes are simple shapes, usually limited torectangles of certain minimum and maximum sizes and with sides which areparallel to the axes of a Cartesian coordinate plane, and triangles withtheir three internal angles being 45 degrees, 45 degrees, and 90 degreesof certain minimum and maximum sizes. At pre-determined locations, dosesof electrons are shot into the resist with these simple shapes. Thetotal writing time for this type of system increases with the number ofshots. In character projection (CP), there is a stencil in the systemthat has in it a variety of apertures or characters which may berectilinear, arbitrary-angled linear, circular, nearly circular,annular, nearly annular, oval, nearly oval, partially circular,partially nearly circular, partially annular, partially nearly annular,partially nearly oval, or arbitrary curvilinear shapes, and which may bea connected set of complex shapes or a group of disjointed sets of aconnected set of complex shapes. An electron beam can be shot through acharacter on the stencil to efficiently produce more complex patterns onthe reticle. In theory, such a system can be faster than a VSB systembecause it can shoot more complex shapes with each time-consuming shot.Thus, an E-shaped pattern shot with a VSB system takes four shots, butthe same E-shaped pattern can be shot with one shot with a characterprojection system. Note that VSB systems can be thought of as a special(simple) case of character projection, where the characters are justsimple characters, usually rectangles or 45-45-90 degree triangles. Itis also possible to partially expose a character. This can be done by,for instance, blocking part of the particle beam. For example, theE-shaped pattern described above can be partially exposed as an F-shapedpattern or an I-shaped pattern, where different parts of the beam arecut off by an aperture. This is the same mechanism as how various sizedrectangles can be shot using VSB. In this disclosure, partial projectionis used to mean both character projection and VSB projection.

As indicated, in optical lithography the lithographic mask or reticlecomprises geometric patterns corresponding to the circuit components tobe integrated onto a substrate. The patterns used to manufacture thereticle may be generated utilizing computer-aided design (CAD) softwareor programs. In designing the patterns the CAD program may follow a setof pre-determined design rules in order to create the reticle. Theserules are set by processing, design, and end-use limitations. An exampleof an end-use limitation is defining the geometry of a transistor in away in which it cannot sufficiently operate at the required supplyvoltage. In particular, design rules can define the space tolerancebetween circuit devices or interconnect lines. The design rules are, forexample, used to ensure that the circuit devices or lines do notinteract with one another in an undesirable manner. For example, thedesign rules are used so that lines do not get too close to each otherin a way that may cause a short circuit. The design rule limitationsreflect, among other things, the smallest dimensions that can bereliably fabricated. When referring to these small dimensions, oneusually introduces the concept of a critical dimension. These are, forinstance, defined as the smallest width of a line or the smallest spacebetween two lines, those dimensions requiring exquisite control.

One goal in integrated circuit fabrication by optical lithography is toreproduce on the substrate the original circuit design by use of thereticle. Integrated circuit fabricators are always attempting to use thesemiconductor wafer real estate as efficiently as possible. Engineerskeep shrinking the size of the circuits to allow the integrated circuitsto contain more circuit elements and to use less power. As the size ofan integrated circuit critical dimension is reduced and its circuitdensity increases, the critical dimension of the circuit pattern orphysical design approaches the resolution limit of the optical exposuretool used in optical lithography. As the critical dimensions of thecircuit pattern become smaller and approach the resolution value of theexposure tool, the accurate transcription of the physical design to theactual circuit pattern developed on the resist layer becomes difficult.To further the use of optical lithography to transfer patterns havingfeatures that are smaller than the light wavelength used in the opticallithography process, a process known as optical proximity correction(OPC) has been developed. OPC alters the physical design to compensatefor distortions caused by effects such as optical diffraction and theoptical interaction of features with proximate features. OPC includesall resolution enhancement technologies performed with a reticle.

OPC may add sub-resolution lithographic features to mask patterns toreduce differences between the original physical design pattern, thatis, the design, and the final transferred circuit pattern on thesubstrate. The sub-resolution lithographic features interact with theoriginal patterns in the physical design and with each other andcompensate for proximity effects to improve the final transferredcircuit pattern. One feature that is used to improve the transfer of thepattern is a sub-resolution assist feature (SRAF). Another feature thatis added to improve pattern transference is referred to as “serifs”.Serifs are small features that can be positioned on a corner of apattern to sharpen the corner in the final transferred image. It isoften the case that the precision demanded of the surface manufacturingprocess for SRAFs is less than that for patterns that are intended toprint on the substrate, often referred to as main features. Serifs are apart of a main feature. As the limits of optical lithography are beingextended far into the sub-wavelength regime, the OPC features must bemade more and more complex in order to compensate for even more subtleinteractions and effects. As imaging systems are pushed closer to theirlimits, the ability to produce reticles with sufficiently fine OPCfeatures becomes critical. Although adding serifs or other OPC featuresto a mask pattern is advantageous, it also substantially increases thetotal feature count in the mask pattern. For example, adding a serif toeach of the corners of a square using conventional techniques adds eightmore rectangles to a mask or reticle pattern. Adding OPC features is avery laborious task, requires costly computation time, and results inmore expensive reticles. Not only are OPC patterns complex, but sinceoptical proximity effects are long range compared to minimum line andspace dimensions, the correct OPC patterns in a given location dependsignificantly on what other geometry is in the neighborhood. Thus, forinstance, a line end will have different size serifs depending on whatis near it on the reticle. This is even though the objective might be toproduce exactly the same shape on the wafer. It is conventional todiscuss the OPC-decorated patterns to be written on a reticle in termsof main features, that is features that reflect the design before OPCdecoration, and OPC features, where OPC features might include serifs,jogs, and SRAF. To quantify what is meant by slight variations, atypical slight variation in OPC decoration from neighborhood toneighborhood might be 5% to 80% of a main feature size. Note that forclarity, variations in the design of the OPC are what is beingreferenced. Manufacturing variations, such as line-edge roughness andcorner rounding, will also be present in the actual surface patterns.When these OPC variations produce substantially the same patterns on thewafer, what is meant is that the geometry on the wafer is targeted to bethe same within a specified error, which depends on the details of thefunction that that geometry is designed to perform, e.g., a transistoror a wire. Nevertheless, typical specifications are in the 2%-50% of amain feature range. There are numerous manufacturing factors that alsocause variations, but the OPC component of that overall error is oftenin the range listed. OPC shapes such as sub-resolution assist featuresare subject to various design rules, such as a rule based on the size ofthe smallest feature that can be transferred to the wafer using opticallithography. Other design rules may come from the mask manufacturingprocess or, if a character projection charged particle beam writer isused to form the pattern on a reticle, from the stencil manufacturingprocess. It should also be noted that the accuracy requirement of theSRAF features on the mask may be lower than the accuracy requirementsfor the main features on the mask.

Inverse lithography technology (ILT) is one type of OPC technique. ILTis a process in which a pattern to be formed on a reticle is directlycomputed from a pattern which is desired to be formed on a substratesuch as a silicon wafer. This may include simulating the opticallithography process in the reverse direction, using the desired patternon the surface as input. ILT-computed reticle patterns may be purelycurvilinear—i.e. completely non-rectilinear—and may include circular,nearly circular, annular, nearly annular, oval and/or nearly ovalpatterns. Since curvilinear patterns are difficult and expensive to formon a reticle using conventional techniques, rectilinear approximationsof the curvilinear patterns may be used. In this disclosure ILT, OPC,source mask optimization (SMO), and computational lithography are termsthat are used interchangeably.

There are a number of technologies used for forming patterns on areticle, including using optical lithography or charged particle beamlithography. If charged particle beam lithography is used, the totalwriting time increases with the number of shots. Reticle writing for themost advanced technology nodes typically involves multiple passes ofcharged particle beam writing, a process called multi-pass exposure,whereby the given shape on the reticle is written and overwritten.Typically, two to four passes are used to write a reticle to average outprecision errors in the charged particle beam writer, allowing thecreation of more accurate photomasks. Also typically, the list of shots,including the dosages, is the same for every pass. In one variation ofmulti-pass exposure, the lists of shots may vary among exposure passes,but the union of the shots in any exposure pass covers the same area.Multi-pass writing can reduce over-heating of the resist coating thesurface. Multi-pass writing also averages out random errors of thecharged particle beam writer. Multi-pass writing using different shotlists for different exposure passes can also reduce the effects ofcertain systemic errors in the writing process.

A layout pattern, such as a post-OPC pattern, must be decomposed orfractured into a set of VSB and/or CP shots so that a charged particlebeam writer can expose the pattern onto a surface using the set or listof shots. Conventional fracturing tools generate a set ofnon-overlapping or disjoint shots of constant dosage, the dosage subjectto later adjustment by proximity effect correction (PEC). Some chargedparticle beam writers require that the pre-PEC shot dosage be constant,since they do not allow dosage assignment on a shot-by-shot basis. Suchcharged particle beam writers perform PEC correction internally afterreading the input shot list. More recently, U.S. patent application Ser.No. 12/202,366, filed Sep. 1, 2008 and entitled “Method and System ForDesign Of A Reticle To Be Manufactured Using Character ProjectionLithography”, and U.S. patent application Ser. No. 12/473,265, filed May27, 2009 and entitled “Method And System For Design Of A Reticle To BeManufactured Using Variable Shaped Beam Lithography” disclose fracturingmethods using overlapping shots.

The cost of charged particle beam lithography is directly related to thetime required to expose a pattern on a surface, such as a reticle orwafer. Conventionally, the exposure time is related to the number ofshots required to produce the pattern. For the most complex integratedcircuit designs, forming the set of layer patterns, either on a set ofreticles or on a substrate, is a costly and time-consuming process.Therefore, there exists a need to reduce the time required to formcomplex patterns, such as curvilinear patterns, on a reticle and othersurfaces, such as by reducing the number of shots required to form thesecomplex patterns and by overcoming shot overlap and shot dosagelimitations of charged particle beam writer systems.

SUMMARY OF THE DISCLOSURE

A method and system for fracturing or mask data preparation or proximityeffect correction is disclosed, wherein a plurality of exposure passes,each with a plurality of shots, is determined. Each exposure pass has abase dosage level, with the base dosage levels being different fordifferent exposure passes.

Methods for manufacturing a reticle and for manufacturing an integratedcircuit are also disclosed, wherein a plurality of charged particle beamexposure passes are used, with base dosage levels being different fordifferent exposure passes.

The methods of the current disclosure can increase the number of dosagelevels with which the reticle or other surface can be exposed when usingsome types of charged particle beam writers. The increased number ofdosage levels can allow for a reduced number of shots required to formpatterns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a character projection charged particle beam system;

FIG. 2A illustrates a single charged particle beam shot and across-sectional dosage graph of the shot;

FIG. 2B illustrates a pair of proximate shots and a cross-sectionaldosage graph of the shot pair;

FIG. 2C illustrates a pattern formed on a resist-coated surface from thepair of FIG. 2B shots;

FIG. 3A illustrates a polygonal pattern;

FIG. 3B illustrates a conventional fracturing of the polygonal patternof FIG. 3A;

FIG. 3C illustrates an alternate fracturing of the polygonal pattern ofFIG. 3A;

FIG. 4A illustrates a square pattern;

FIG. 4B illustrates a pattern resulting from OPC processing of the FIG.4A square pattern;

FIG. 4C illustrates a conventional fracturing of the pattern of FIG. 4B;

FIG. 4D illustrates an exemplary fracturing of the pattern of FIG. 4B;

FIG. 5A illustrates shots from each of two exposure passes;

FIG. 5B illustrates three dosage values that may be obtained from nomore than one shot from each of the two exposure passes of FIG. 5A;

FIG. 5C illustrates five dosage values that may be obtained from no morethan two shots among the two exposure passes of FIG. 5A;

FIG. 5D illustrates four dosage values that may be obtained by usingthree shots among the two exposure passes of FIG. 5A;

FIG. 6A illustrates shots from each of two exposure passes;

FIG. 6B illustrates three dosage values that may be obtained usingexactly two shots among the two exposure passes of FIG. 6A;

FIG. 7A illustrates four dosage values that may be obtained with twoexposure passes, using two shot dosages for each exposure pass;

FIG. 7B illustrates eight dosage values that may be obtained with twoexposure passes, using overlapping shots from each exposure pass;

FIG. 7C illustrates eight dosage values that may be obtained with twoexposure passes, using shots from only one of the exposure passes for asingle area;

FIG. 8A illustrates a curvilinear pattern;

FIG. 8B illustrates a prior art method of forming the pattern of FIG. 8Ausing overlapping shots;

FIG. 8C illustrates non-overlapping shots from a first of two passesthat together can form the pattern of FIG. 8A, using an exemplary methodof the current disclosure;

FIG. 8D illustrates non-overlapping shots from a second of two passesthat together can form the pattern of FIG. 8A, using an exemplary methodof the current disclosure;

FIG. 9A illustrates unassigned dosage shots from a first of two exposurepasses that together can form the pattern of FIG. 8A, using an exemplarymethod of the current disclosure;

FIG. 9B illustrates unassigned dosage shots from a second of twoexposure passes that together can form the pattern of FIG. 8A, using anexemplary method of the current disclosure; and

FIG. 10 illustrates a conceptual flow diagram for manufacturing areticle and photomask, or for exposing a substrate, using an exemplarymethod of the current disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes generating and exposing a series ofshaped beam charged particle beam shots to form a desired pattern on asurface. The shots are written in multiple exposure passes, wherein anyone or more of the following is true:

-   -   The base dosage levels of different exposure passes may be        different;    -   The sum of the base dosage levels of all the exposure passes may        be different from a normal dosage; or    -   The union of shot outlines from one exposure pass may be        different than the union of shot outlines from a different        exposure pass.

Referring now to the drawings, wherein like numbers refer to like items,FIG. 1 illustrates an embodiment of a conventional lithography system100, such as a charged particle beam writer system, in this case anelectron beam writer system, that employs character projection tomanufacture a surface 130. The electron beam writer system 100 has anelectron beam source 112 that projects an electron beam 114 toward anaperture plate 116. The plate 116 has an aperture 118 formed thereinwhich allows the electron beam 114 to pass. Once the electron beam 114passes through the aperture 118 it is directed or deflected by a systemof lenses (not shown) as electron beam 120 toward another rectangularaperture plate or stencil mask 122. The stencil 122 has formed therein anumber of openings or apertures 124 that define various types ofcharacters 126. Each character 126 formed in the stencil 122 may be usedto form a pattern 148 on a surface 130 of a substrate 132, such as asilicon wafer, a reticle or other substrate. In partial exposure,partial projection, partial character projection, or variable characterprojection, electron beam 120 may be positioned so as to strike orilluminate only a portion of one of the characters 126, thereby forminga pattern 148 that is a subset of character 126. For each character 126that is smaller than the size of the electron beam 120 defined byaperture 118, a blanking area 136, containing no aperture, is designedto be adjacent to the character 126, so as to prevent the electron beam120 from illuminating an unwanted character on stencil 122. An electronbeam 134 emerges from one of the characters 126 and passes through anelectromagnetic or electrostatic reduction lens 138 which reduces thesize of the pattern from the character 126. In commonly availablecharged particle beam writer systems, the reduction factor is between 10and 60. The reduced electron beam 140 emerges from the reduction lens138, and is directed by a series of deflectors 142 onto the surface 130as the pattern 148, which is depicted as being in the shape of theletter “H” corresponding to character 126A. The pattern 148 is reducedin size compared to the character 126A because of the reduction lens138. The pattern 148 is drawn by using one shot of the electron beamsystem 100. This reduces the overall writing time to complete thepattern 148 as compared to using a variable shape beam (VSB) projectionsystem or method. Although one aperture 118 is shown being formed in theplate 116, it is possible that there may be more than one aperture inthe plate 116. Although two plates 116 and 122 are shown in thisexample, there may be only one plate or more than two plates, each platecomprising one or more apertures.

In conventional charged particle beam writer systems the reduction lens138 is calibrated to provide a fixed reduction factor. The reductionlens 138 and/or the deflectors 142 also focus the beam on the plane ofthe surface 130. The size of the surface 130 may be significantly largerthan the maximum beam deflection capability of the deflection plates142. Because of this, patterns are normally written on the surface in aseries of stripes. Each stripe contains a plurality of sub-fields, wherea sub-field is within the beam deflection capability of the deflectionplates 142. The electron beam writer system 100 contains a positioningmechanism 150 to allow positioning the substrate 132 for each of thestripes and sub-fields. In one variation of the conventional chargedparticle beam writer system, the substrate 132 is held stationary whilea sub-field is exposed, after which the positioning mechanism 150 movesthe substrate 132 to the next sub-field position. In another variationof the conventional charged particle beam writer system, the substrate132 moves continuously during the writing process. In this variationinvolving continuous movement, in addition to deflection plates 142,there may be another set of deflection plates (not shown) to move thebeam at the same speed and direction as the substrate 132 is moved.

The minimum size pattern that can be projected with reasonable accuracyonto a surface 130 is limited by a variety of short-range physicaleffects associated with the electron beam writer system 100 and with thesurface 130, which normally comprises a resist coating on the substrate132. These effects include forward scattering, Coulomb effect, andresist diffusion. Beam blur is a term used to include all of theseshort-range effects. The most modern electron beam writer systems canachieve an effective beam blur in the range of 20 nm to 30 nm. Forwardscattering may constitute one quarter to one half of the total beamblur. Modern electron beam writer systems contain numerous mechanisms toreduce each of the constituent pieces of beam blur to a minimum. Someelectron beam writer systems may allow the beam blur to be varied duringthe writing process, from the minimum value available on an electronbeam writing system to one or more larger values.

The shot dosage of a charged particle beam writer such as an electronbeam writer system is a function of the intensity of the beam source 112and the exposure time for each shot. Typically the beam intensityremains fixed, and the exposure time is varied to obtain variable shotdosages. The exposure time may be varied to compensate for variousproximity effects, some longer-range and some shorter-range, in aprocess called proximity effect correction (PEC). Electron beam writersystems usually allow setting an overall dosage, called a base dosage,that affects all shots in an exposure pass. Some electron beam writersystems perform dosage compensation calculations within the electronbeam writer system itself, and do not allow the dosage of each shot tobe assigned individually as part of the input shot list, the input shotstherefore having unassigned shot dosages. In such electron beam writersystems all shots have the base dosage, before proximity effectscorrection. Other electron beam writer systems do allow dosageassignment on a shot-by-shot basis. In electron beam writer systems thatallow shot-by-shot dosage assignment, the number of available dosagelevels may be 64 to 4096 or more, or there may be a relatively fewavailable dosage levels, such as 3 to 8 levels. Some embodiments of thecurrent invention are targeted for use with charged particle beamwriting systems which either do not allow dosage assignment on ashot-by-shot basis, or which allow assignment of one of a relatively fewdosage levels.

FIGS. 2A-B illustrate how energy is registered on a resist-coatedsurface from one or more charged particle beam shots. In FIG. 2Arectangular pattern 202 illustrates a shot outline, which is a patternthat will be produced on a resist-coated surface from a shot which isnot proximate to other shots. In dosage graph 210, dosage curve 212illustrates the cross-sectional dosage along a line 204 through shotoutline 202. Line 214 denotes the resist threshold, which is the dosageabove which the resist will register a pattern. As can be seen fromdosage graph 210, dosage curve 212 is above the resist threshold betweenthe X-coordinates “a” and “b”. Coordinate “a” corresponds to dashed line216, which denotes the left-most extent of the shot outline 202.Similarly, coordinate “b” corresponds to dashed line 218, which denotesthe right-most extent of the shot outline 202. The shot dosage for theshot in the example of FIG. 2A is a normal dosage, as marked on dosagegraph 210. In conventional mask writing methodology, the normal dosageis set so that a relatively large rectangular shot will register apattern of the desired size on the resist-coated surface. The normaldosage therefore depends on the value of the resist threshold 214.

FIG. 2B illustrates the shot outlines of two particle beam shots, andthe corresponding dosage curve. Shot outline 222 and shot outline 224result from two proximate particle beam shots. In dosage graph 220,dosage curve 230 illustrates the dosage along the line 226 through shotoutlines 222 and 224. As shown in dosage curve 230, the dosageregistered by the resist along line 226 is the combination, such as thesum, of the dosages from two particle beam shots, represented by shotoutline 222 and shot outline 224. As can be seen, dosage curve 230 isabove the threshold 214 from X-coordinate “a” to X-coordinate “d”. Thisindicates that the resist will register the two shots as a single shape,extending from coordinate “a” to coordinate “d”. FIG. 2C illustrates apattern 252 that the two shots from the example of FIG. 2B may form.

When using conventional non-overlapping shots using a single exposurepass, conventionally all shots are assigned a normal dosage before PECdosage adjustment. A charged particle beam writer which does not supportshot-by-shot dosage assignment can therefore be used by setting the basedosage to a normal dosage. If multiple exposure passes are used withsuch a charged particle beam writer, the base dosage is conventionallyset according to the following equation:

base dosage=normal dosage/# of exposure passes

FIGS. 3A-C illustrate two known methods of fracturing a polygonalpattern. FIG. 3A illustrates a polygonal pattern 302 that is desired tobe formed on a surface. FIG. 3B illustrates a conventional method offorming this pattern using non-overlapping or disjoint shots. Shotoutline 310, shot outline 312 and shot outline 314 are mutuallydisjoint. Additionally, the three shots associated with these shotoutlines all use a desired normal dosage, before proximity correction.An advantage of using the conventional method as shown in FIG. 3B isthat the response of the resist can be easily predicted. Also, the shotsof FIG. 3B can be exposed using a charged particle beam system whichdoes not allow dosage assignment on a shot-by-shot basis, by setting thebase dosage of the charged particle beam writer to the normal dosage.FIG. 3C illustrates an alternate method of forming the pattern 302 on aresist-coated surface using overlapping shots, disclosed in U.S. patentapplication Ser. No. 12/473,265, filed May 27, 2009 and entitled “MethodAnd System For Design Of A Reticle To Be Manufactured Using VariableShaped Beam Lithography.” In FIG. 3C the constraint that shot outlinescannot overlap has been eliminated, and shot 320 and shot 322 dooverlap. In the example of FIG. 3C, allowing shot outlines to overlapallows forming the pattern 302 in only two shots, compared to the threeshots of FIG. 3B. In FIG. 3C, however the response of the resist to theoverlapping shots is not as easily predicted as in FIG. 3B. Inparticular, the interior corners 324, 326, 328 and 330 may register asexcessively rounded because of the large dosage received by region 332.Charged particle beam simulation may be used to determine the pattern332 registered by the resist. In one embodiment, charged particle beamsimulation may be used to calculate the dosage for each grid location ina two-dimensional (X and Y) grid, creating a grid of calculated dosagescalled a dosage map. The results of charged particle beam simulation mayindicate use of non-normal dosages for shot 320 and shot 322.

FIGS. 4A-D illustrate examples of various known ways of forming acurvilinear pattern on a resist-covered surface. FIG. 4A illustrates anexample of a square pattern 402, such as a contact or via for anintegrated circuit design, that may be desired to be formed on aresist-coated surface. A large integrated circuit may contain millionsof contact shapes. Pattern 402 is an original design pattern, before OPCprocessing. If advanced OPC processing, such as ILT, is performed onpattern 402, a pattern 404 of FIG. 4B may result. FIG. 4C illustrates anexample of a conventional method for forming the pattern 404 using a setof non-overlapping, normal-dosage VSB shots 406. Shot set 406 shows theshot outlines of seven shots, including both rectangular and triangularVSB shots. The pattern registered by shot set 406 may not match thedesired pattern 404 very closely, but the use of more than seven shotsper contact may be considered impractical. FIG. 4D illustrates shotoutlines of an example of three overlapping shots that can moreaccurately form pattern 404 than can the shot set 406. This method ofoverlapping shots is disclosed in the aforementioned U.S. patentapplication Ser. No. 12/473,265. In the set of three shots, shot 410 andshot 412 has a normal dosage, and shot 414 has a dosage of 0.6 times anormal dosage. Charged particle beam simulation may be used to determinethe result that will be registered by the resist using shots 410, 412and 414. The solution of FIG. 4D, however, cannot be conventionallyimplemented when using a charged particle beam writer which does notallow dosage assignment on a per-shot basis. If the base dosage for thewriting system is set to a normal dosage divided by a predeterminednumber of exposure passes, then shot 410 and shot 412 can be made withthe proper total dosage, but shot 414 cannot be made. Neither can thesolution of FIG. 4D be conventionally implemented when using a chargedparticle beam writer which does not allow shots to overlap.

To overcome limitations presented by a charged particle beam writer thatdoes not allow dosage to be assigned for each shot, the currentinvention novelly enables the solution of FIG. 4D to be implemented byusing multiple exposure passes. For example, for FIG. 4D, a solution canbe obtained using two exposure passes. Shot 410 and shot 412 may bewritten in one exposure pass which has a base dosage level of 1.0 timesa normal dosage, and shot 414 may be written in another exposure passwhich has a base dosage of 0.6 times a normal dosage. In addition to thebase dosages being different between the two exposure passes, it shouldbe noted that the sum of base dosage levels from the two exposure passesdoes not equal a normal dosage, unlike with a conventional multi-passexposure technique. Additionally, the union of the shots for the twoexposure passes is different, with one exposure pass including shot 410and shot 412 and a different exposure pass including shot 414.

When using multiple exposure passes, each exposure pass after the firstpass adds an overhead in total write time. This forms a practical limitto the number of exposure passes. When using a charged particle beamwriter which either does not allow dosage assignment on a per-shotbasis, or which provides a very few available shot dosages, it istherefore advantageous to maximize the number of available shot dosagesusing the fewest number of exposure passes. FIGS. 5A-D illustrate anexample of how a multi-pass writing technique may be used to expose aresist-coated surface with multiple dosage values, using a particle beamwriter that does not allow dosage assignment on a per-shot basis. FIG.5A illustrates a single shot outline 502 from an exposure pass calledpass “A”, and a single shot outline 504 from an exposure pass calledpass “B”. Shots in pass “A” have an assigned dosage of 0.4 times anormal dosage, and shots in pass “B” have an assigned dosage of 0.5times a normal dosage. In the most restrictive case, the chargedparticle beam writer may not allow shots to overlap within an exposurepass. FIG. 5B illustrates that three total dosages are available in thismost-restrictive case, using two exposure passes:

-   -   0.4 times normal dosage using one shot from pass “A” only,        illustrated by shot outline 506;    -   0.5 times normal dosage using one shot from pass “B” only,        illustrated by shot outline 508; and    -   0.9 times normal dosage using one shot from pass “A” and one        superimposed shot from pass “B”, illustrated by the pair of        overlapping shot outlines 510.

FIG. 5C illustrates that five total dosages are available using twoshots in a less-restrictive case where the charged particle beam writerallows shots to overlap within an exposure pass:

-   -   0.4 times normal dosage using one shot from pass “A” only,        illustrated by shot outline 520;    -   0.5 times normal dosage using one shot from pass “B” only,        illustrated by shot outline 522;    -   0.8 times normal dosage using two overlapping shots from pass        “A”, illustrated by the pair of overlapping shot outlines 524;    -   0.9 times normal dosage using one shot from pass “A” and one        shot from pass “B”, illustrated by the pair of overlapping shot        outlines 526; and    -   1.0 times normal dosage using two overlapping shots from pass        “B”, illustrated by the pair of overlapping shot outlines 528.

FIG. 5D illustrates four additional dosages that are available by usingthree overlapping shots:

-   -   1.2 times normal dosage using three shots from pass “A”,        illustrated by the trio of overlapping shot outlines 530;    -   1.5 times normal dosage using three shots from pass “B”,        illustrated by the trio of overlapping shot outlines 532;    -   1.3 times normal dosage using two shots from pass “A” and one        shot from pass “B”, illustrated by the trio of overlapping shot        outlines 534; and    -   1.4 times normal dosage using one shot from pass “A” and two        shots from pass “B”, illustrated by the trio of overlapping shot        outlines 536.

Larger numbers of dosage values may be obtained using three or moreexposure passes. It should be noted that some of the conventional goalsof multi-pass exposure—i.e. accuracy-improvement—may still be achievedwhen a plurality of shots having a similar dosage are used, such as inshot pair 510, shot pair 524, shot pair 526 or shot pair 528. In thesefour shot pairs, the dosages of the two shots in the shot pair arewithin 35% of each other, constituting similar dosages. Single shots,such as shot 506, shot 508, shot 520 or shot 522 do not gain anyaccuracy improvement, since the surface is exposed during only one ofthe two passes. Additional multi-exposure accuracy improvement, ifneeded, can be obtained by combining conventional multi-pass exposurewith the technique disclosed above, using additional exposure passes.

FIGS. 6A-B illustrate another example of using multi-pass exposure. FIG.6A illustrates shot 602 from a pass “A” which has a base dosage level of0.35 times a normal dosage, and shot 604 from a pass “B” which has abase dosage level of 0.50 times a normal dosage. FIG. 6B illustratesthree shot combinations of similar-dosage shots, wherein a reduction insome kinds of writing accuracy errors can be obtained. Shot combination606 consists of two superimposed shots from pass “A”, totaling a dosageof 0.7 times a normal dosage. Shot combination 608 consists of twosuperimposed shots, one from pass “A” and one from pass “B”, totaling adosage of 0.85 times a normal dosage. Shot combination 610 consists oftwo superimposed shots from pass “B”, totaling a dosage of 1.0 times anormal dosage. FIG. 6B illustrates how multiple dosages may be deliveredto the resist-coated surface, while still gaining the some of theaccuracy-improvement benefit of conventional multi-pass exposure.

FIGS. 7A-C illustrate another embodiment of the current disclosure inwhich multiple exposure passes may be used to increase the number ofdosage levels available when using a charged particle beam writer whichsupports a small number of shot dosages. In the FIGS. 7A-C example, thecharged particle beam writer allows a shot within an exposure pass tohave one of two dosage levels, the shot dosage levels being expressed inthis example as a fractional multiple of the base dosage level. In otherembodiments, the charged particle beam writer may allow more than twoshot dosage levels, such as 4, 8 or 16 shot dosage levels. Also, inother embodiments the shot dosages may be expressed in other ways, suchas by an absolute actual dosage which includes effects of the basedosage. Two exposure passes are illustrated in the FIGS. 7A-C example,with exposure pass “A” having a base dosage level of 0.4 times a normaldosage, and exposure pass “B” having a base dosage level of 0.5 times anormal dosage. FIG. 7A illustrates the dosages which are available usinga single shot. The shot dosages available within exposure pass “A” areshot 702 with a shot dosage multiplier of 1.0, and shot 704 with a shotdosage multiplier of 0.7. The actual dosage for shot 702 is the basedosage times the shot multiplier, or 0.4*1.0=0.40 times a normal dosage.Similarly, the actual dosage for shot 704 is 0.4*0.7=0.28 times a normaldosage. Exposure pass “B” has a base dosage level of 0.5 times a normaldosage. The shots available within exposure pass “B” are shot 712, witha shot multiplier of 1.0, and shot 714 with a shot multiplier of 0.7.The actual dosage for shot 712 is the base dosage times the shotmultiplier, or 0.5*1.0=0.50 times a normal dosage. Similarly, the actualdosage for shot 714 is 0.5*0.7=0.35 times a normal dosage. Note that inthe example of FIG. 7A the sum of the base dosage levels is less than anormal dosage. A total of four shot dosages are therefore available withthe two passes, using a single shot total between the two passes:

-   -   Shot 712: 0.50 times normal dosage    -   Shot 702: 0.40 times normal dosage    -   Shot 714: 0.35 times normal dosage    -   Shot 704: 0.28 times normal dosage        These shots may typically be used in overlapping combinations,        including partially overlapping combinations, to form a pattern        on a resist-covered surface.

FIG. 7B illustrates eight dosages that may be obtained by overlapping atleast two shots from FIG. 7A, using at least one shot from pass “A” andat least one shot from pass “B”:

-   -   Shot 720 from pass “A” with a shot multiplier of 1.0 and shot        721 from pass “B” with a shot multiplier of 1.0, for a total        dosage 722 of (0.4*1.0)+(0.5*1.0)=0.9 times a normal dosage;    -   Shot 724 from pass “A” with a shot multiplier of 1.0 and shot        725 from pass “B” with a shot multiplier of 0.7, for a total        dosage 726 of (0.4*1.0)+(0.5*0.7)=0.75 times a normal dosage;    -   Shot 728 from pass “A” with a shot multiplier of 0.7 and shot        729 from pass “B” with a shot multiplier of 1.0, for a total        dosage 730 of (0.4*0.7)+(0.5*1.0)=0.78 times a normal dosage;    -   Shot 732 from pass “A” with a shot multiplier of 0.7 and shot        733 from pass “B” with a shot multiplier of 0.7, for a total        dosage 734 of (0.4*0.7)+(0.5*0.70=0.63 times a normal dosage;    -   Shots 736 and 737 from pass “A”, both shots with a shot        multiplier of 0.7, and shot 738 from pass “B” with a shot        multiplier of 1.0, for a total dosage 739 of        (0.4*0.7)+(0.4*0.7)+(0.5*1.0)=1.06 times a normal dosage;    -   Shots 741 and 742 from pass “A”, both shots with a shot        multiplier of 0.7, and shot 743 from pass “B” with a shot        multiplier of 0.7, for a total dosage 744 of        (0.4*0.7)+(0.4*0.7)+(0.5*0.7)=0.91 times a normal dosage;    -   Shot 746 from pass “A” with a shot multiplier of 1.0 and shots        747 and 748 from pass “B”, both pass “B” shots with a shot        multiplier of 0.7, for a total dosage 749 of        (0.4*1.0)+(0.5*0.7)+(0.5*0.7)=1.10 times a normal dosage; and    -   Shot 751 from pass “A” with a shot multiplier of 0.7 and shots        752 and 753 from pass “B”, both pass “B” shots with a shot        multiplier of 0.7, for a total dosage 754 of        (0.4*0.7)+(0.5*0.7)+(0.5*0.7)=0.98 times a normal dosage.        The FIG. 7B shot combinations all include at least one shot from        each exposure pass. This provides at least some of the accuracy        improvement benefit of conventional multi-pass writing.

FIG. 7C illustrates even more shot combinations of the FIG. 7A shots,but where shots from only one exposure pass are overlapped, therebyproviding less accuracy improvement benefit than the shot combinationsof FIG. 7B. FIG. 7C combinations include:

-   -   Shot 765 from pass “A” with a shot multiplier of 1.0 and shot        766 from pass “A” with a shot multiplier of 1.0, for a total        dosage 767 of (0.4*1.0)+(0.4*1.0)=0.80 times a normal dosage;    -   Shot 768 from pass “A” with a shot multiplier of 1.0 and shot        769 from pass “A” with a shot multiplier of 0.7, for a total        dosage 770 of (0.4*1.0)+(0.4*0.7)=0.68 times a normal dosage;    -   Shot 772 from pass “A” with a shot multiplier of 0.7 and shot        773 from pass “A” with a shot multiplier of 0.7, for a total        dosage 774 of (0.4*0.7)+(0.4*07)=0.56 times a normal dosage;    -   Shot 775 from pass “A” with a shot multiplier of 0.7, and shot        776 from pass “A” with a shot multiplier of 0.7, and shot 777        from pass “A” with a shot multiplier of 0.7, for a total dosage        778 of (0.4*0.7)+(0.4*0.7)+(0.4*0.7)=0.84 times a normal dosage;    -   Shot 785 from pass “B” with a shot multiplier of 1.0 and shot        786 from pass “B” with a shot multiplier of 1.0, for a total        dosage 787 of (0.5*1.0)+(0.5*1.0)=1.0 times a normal dosage;    -   Shot 788 from pass “B” with a shot multiplier of 1.0 and shot        789 from pass “B” with a shot multiplier of 0.7, for a total        dosage 790 of (0.5*1.0)+(0.5*0.7)=0.85 times a normal dosage;    -   Shot 792 from pass “B” with a shot multiplier of 0.7 and shot        793 from pass “B” with a shot multiplier of 0.7, for a total        dosage 794 of (0.5*0.7)+(0.5*0.7)=0.70 times a normal dosage;        and    -   Shot 795 from pass “B” with a shot multiplier of 0.7, and shot        796 from pass “B” with a shot multiplier of 0.7, and shot 797        from pass “B” with a shot multiplier of 0.7, for a total dosage        798 of (0.5*0.7)+(0.5*0.7)+(0.5*0.7)=1.05 times a normal dosage.        The availability of shots with a wider dosage variation may        allow a reduction in the total shot count required to form a        pattern on a resist-coated surface. As the FIGS. 7A-C example        illustrates, the use of multiple exposure passes can mulitply        the number of shot dosage levels available.

FIGS. 8A-D illustrate the use of multiple exposure passes to form acurvilinear pattern. FIG. 8A illustrates a curvilinear pattern 800 thatis desired to be formed on a resist-covered surface. Pattern 800 is amostly-constant-width curvilinear path or track, with a bump 802 on theupper side. FIG. 8B illustrates a combination of prior art methods forforming the pattern 802:

-   -   The constant-width path or track can be formed with a series of        overlapping circular CP shots, nine in this example, consisting        of shot 812, shot 814, shot 816, shot 818, shot 820, shot 822,        shot 824, shot 826 and shot 828. This method is disclosed in        U.S. patent application Ser. No. 12/618,722 filed Nov. 14, 2009,        entitled “Method For Fracturing And Forming A Pattern Using        Curvilinear Characters With Charged Particle Beam Lithography.”    -   The bump 802 is formed using a differently sized circular CP        shot 830 which overlaps shot 822. The use of overlapping CP        shots to form a pattern is disclosed in U.S. patent application        Ser. No. 12/202,364 filed Sep. 1, 2008, entitled “Method And        System For Manufacturing A Reticle Using Character Projection        Lithography.”        The prior art methods illustrated in FIG. 8B require use of a        charged particle beam writer that allows overlapping shots. FIG.        8C and FIG. 8D illustrate an exemplary method of how the pattern        800 may be formed using a charged particle beam writer which        does not allow shot overlap within an exposure pass, by using        two exposure passes, according to the current disclosure, with        the two exposure passes in this example being called pass “A”        and pass “B”. FIG. 8C illustrates outlines of a set of six pass        “A” shots 840, including shot 842, shot 844, shot 846, shot 848        and shot 850, each of these shots using an assigned shot        multiplier of 0.5. Also shown is shot outline 852, with this        shot having a shot multiplier of 1.0. The dosage of shot 852 is        higher than the other pass “A” shots, because no pass “B” shot        outline overlaps the top of the bump 802. FIG. 8D illustrates        outlines of a set of four pass “B” shots 860, including shot        862, shot 864, shot 866 and shot 868. The outlines of shots from        pass “A” are shown in dashed lines, so that overlap between pass        “A” shot outlines and pass “B” shot outlines can be seen. As can        be seen in FIG. 8D, the union of dashed shot outlines for pass        “A” shots is different from the union of solid shot outlines for        pass “B” shots. Note that pass “B” cannot have a shot        corresponding to shot 852 of pass “A”, for such a shot would        overlap pass “B” shot 866, as can be seen. FIGS. 8C-D illustrate        how the shot count reduction benefits of using overlapping shots        can be obtained, even when using a charged particle beam writer        which does not allow shot overlap within an exposure pass.

FIGS. 9A-B illustrate an exemplary method for forming pattern 800 usinga charged particle beam writer which does not allow shot-by-shot dosageassignment, according to the current disclosure. The example of FIGS.9A-B uses two exposure passes, called pass “A” and pass “B”. FIG. 9Aillustrates outlines of a set 900 of six pass “A” shots, including shot902, shot 904, shot 906, shot 908, shot 910 and shot 912. A pass “A”base dosage of 0.5 times a normal dosage is used in this example. FIG.9B illustrates outlines of a set 920 of five pass “B” shots, includingshot 922, shot 924, shot 926, shot 928 and shot 930. FIG. 9B also showsthe shots from pass “A” in dashed lines, so that the overlap betweenpass “A” shots and pass “B” shots may be seen. A pass “B” base dosage of0.5 times a normal dosage is used in this example. As can be seen inFIG. 9B, the union of dashed shot outlines for pass “A” shots isdifferent from the union of solid shot outlines for pass “B” shots. Notethat shot 930 completely overlaps shot 912, leading to a total dosage of1.0 times a normal dosage for the bump area 802 of the pattern 800. Inthis example, the sum of the base dosages for both exposure passes is0.5+0.5=1.0 times a normal dosage. In another embodiment, the pass “A”base dosage may be 0.6 times a normal dosage and the pass “B” basedosage may be 0.4 times a normal dosage, the sum of the dosages for thetwo passes therefore also being 1.0 times a normal dosage. In otherembodiments, the sum of the base dosages for all the exposure passes maynot equal 1.0. For example, two exposure passes may have base dosages of0.6 and 0.6 times a normal dosage, the sum of the base dosages for thetwo exposure passes being 1.2 times a normal dosage. In another example,two exposure passes may have base dosages of 0.6 and 0.7 times a normaldosage, the sum of the base dosages for the two exposure passes being1.3 times a normal dosage. A benefit of having the sum of the dosagesfor all the exposure passes equal to 1.0 time a normal dosage is thatthe set of exposure passes may also be used with conventionalfracturing. This allows part of the surface to be fracturedconventionally, and the rest of the surface to be fractured withoverlapping shots, shots with varying assigned dosages or by usingdifferent shot lists for different exposure passes, where the union ofthe shots for different exposure passes is different. FIGS. 9A-Billustrate how shot combinations of varying dosages can be written usinga charged particle beam writer which does not provide for shot-by-shotdosage assignment.

The dosage that would be received by a surface can be calculated andstored as a two-dimensional (X and Y) dosage map called a glyph. Atwo-dimensional dosage map or glyph is a two-dimensional grid ofcalculated dosage values for the vicinity of the shot or shotscomprising the glyph. In some embodiments the dosage map grid may beuniform, while in other embodiments, the dosage map grid may benon-uniform. The calculated dosage map or glyph and the list of shotscomprising the glyph can be stored in a library of glyphs. The glyphlibrary can be used as input during fracturing of the patterns in adesign. For example, referring again to FIG. 4D, a dosage map may becalculated from the series of shots including shot 410, shot 412 andshot 414, and stored in the glyph library. If during fracturing, one ofthe input patterns is a pattern of the same shape as pattern 404, thenthe shots comprising the glyph may be retrieved from the library,avoiding the computational effort of determining an appropriate set ofshots to form the input pattern. A series of glyphs may also be combinedto create a parameterized glyph. Parameters may be discrete or may becontinuous.

FIG. 10 illustrates an exemplary conceptual flow diagram 1000 of amethod for forming a pattern on a surface according to the currentdisclosure. There are four types of input data to the process: stencilinformation 1018, which is information about the CP characters, if any,on the stencil of the charged particle beam writer; process information1036, which includes information such as the resist dosage thresholdabove which the resist will register a pattern; pre-determined exposureparameters 1060, such as the number of exposure passes and the basedosage levels for each pass; and a computer representation of thedesired pattern 1016 to be formed on the surface. Parameters 1060 may begiven as input, or may be calculated automatically given a pattern 1016.In addition, initial optional steps 1002-1012 involve the creation of alibrary of glyphs. The first step in the optional creation of a libraryof glyphs is VSB/CP shot selection 1002, in which one or more VSB or CPshots, each shot either with or without an assigned dosage, are combinedto create a set of shots 1004. The set of shots 1004 may includeoverlapping VSB shots and/or overlapping CP shots. Shots in the set ofshots may also have a beam blur specified. The VSB/CP shot selectionstep 1002 uses the stencil information 1018, which includes informationabout the CP characters that are available on the stencil. The set ofshots 1004 is simulated in step 1006 using charged particle beamsimulation to create a dosage map 1008 of the set of shots. Step 1006may include simulation of various physical phenomena including forwardscattering, resist diffusion, Coulomb effect, etching, fogging, loading,resist charging, and backward scattering. The result of step 1006 is atwo-dimensional dosage map 1008 which represents the combined dosagefrom the set of shots 1004 at each of the grid positions in the map. Thedosage map 1008 is called a glyph. In step 1010 the information abouteach of the shots in the set of shots, and the dosage map 1008 of thisadditional glyph is stored a library of glyphs 1012. In one embodiment,a set of glyphs may be combined into a type of glyph called aparameterized glyph.

The required portion of the flow 1000 involves writing a pattern to asurface, such as a silicon wafer or a reticle used for creation of aphotomask. In step 1020 a combined dosage map for the surface or for aportion of the surface is calculated. Step 1020 uses as input thedesired pattern 1016 to be formed on the surface, the processinformation 1036, the pre-determined exposure parameters 1060, thestencil information 1018, and the glyph library 1012 if a glyph libraryhas been created. In step 1020 an initial surface dosage map may becreated, into which the shot dosage maps will be combined. Initially,the surface dosage map contains no shot dosage map information. In oneembodiment, the grid squares of the surface dosage map may beinitialized with an estimated correction for long-range effects such asbackscattering, fogging, or loading, a term which refers to the effectsof localized resist developer depletion. Step 1020 may involve VSB/CPshot selection 1022, or glyph selection 1034, or both of these. Step1020 may also involve assignment of a shot to one of a plurality ofexposure passes. If shot dosages are allowed, then the shot dosage maybe expressed as a fractional multiple of the base dosage. If a VSB or CPshot is selected, the shot is simulated using charged particle beamsimulation in step 1024 and a dosage map 1026 of the shot is created.The charged particle beam simulation may comprise convolving a shapewith a Gaussian. The convolution may be with a binary function of theshape, where the binary function determines whether a point is inside oroutside the shape. The shape may be an aperture shape or multipleaperture shapes, or a slight modification thereof. In one embodiment,this simulation may include looking up the results of a previoussimulation of the same shot, such as when using a temporary shot dosagemap cache. A higher-than-minimum beam blur may be specified for the VSBor CP shot. Both VSB and CP shots may be allowed to overlap, and mayhave varying dosages with respect to each other, subject to constraintby the predetermined exposure parameters 1060. If a glyph is selected,the dosage map of the glyph is input from the glyph library. In step1020, the various dosage maps of the shots and/or glyphs are combinedinto the surface dosage map. In one embodiment, the combination is doneby adding the dosages. Using the resulting combined dosage map, thepredetermined exposure parameters 1060, and the process information 1036containing resist characteristics, a surface pattern may be calculated.If the calculated surface pattern matches the desired pattern 1016within a pre-determined tolerance, then a combined shot list 1038 isoutput, containing the determined VSB/CP shots and the shotsconstituting the selected glyphs. If the calculated surface pattern doesnot match the target pattern 1016 within a predetermined tolerance ascalculated in step 1020, the set of selected CP shots, VSB shots and/orglyphs may be revised, the dosage maps may be recalculated, and thesurface pattern may be recalculated. In one embodiment, the initial setof shots and/or glyphs may be determined in a correct-by-constructionmethod, so that no shot or glyph modifications are required. In anotherembodiment, step 1020 includes an optimization technique so as tominimize either the total number of shots represented by the selectedVSB/CP shots and glyphs, or the total charged particle beam writingtime, or some other parameter. In yet another embodiment, VSB/CP shotselection 1022 and glyph selection 1034 are performed so as to generatemultiple sets of shots, each of which can form a surface image thatmatches the desired pattern 1016, but at a lower-than-normal dosage, tosupport multi-pass writing.

The combined shot list 1038 comprises the determined list of selectedVSB shots, selected CP shots and shots constituting the selected glyphs.The shots in the final shot list 1038 may include assigned dosages, ormay have unassigned shot dosages. Shots may also include a beam blurspecification. In step 1040, proximity effect correction (PEC) and/orother corrections may be performed or corrections may be refined fromearlier estimates. Thus, step 1040 uses the combined shot list 1038 asinput and produces a final shot list 1042 in which the shot dosages havebeen adjusted, either from the assigned shot dosage or, for unassigneddosage shots, the base dosage. The group of steps from step 1020 throughstep 1042, or subsets of this group of steps, are collectively calledfracturing or mask data preparation. The final shot list 1042 is used bythe charged particle beam writer in step 1044 to expose resist withwhich the surface has been coated, thereby forming a pattern 1046 on theresist. In step 1048 the resist is developed. In the case where thesurface is the surface of a wafer, the development of the resist forms apattern 1054 on the wafer surface. In the case where the surface is areticle, further processing steps 1050 are performed to transform thereticle with the pattern into a photomask 1052.

The fracturing, mask data preparation, proximity effect correction, andpattern writing flows described in this disclosure may be implementedusing general-purpose computers with appropriate computer software ascomputation devices. Due to the large amount of calculations required,multiple computers or processor cores may also be used in parallel. Inone embodiment, the computations may be subdivided into a plurality of2-dimensional geometric regions for one or more computation-intensivesteps in the flow, to support parallel processing. In anotherembodiment, a special-purpose hardware device, either used singly or inmultiples, may be used to perform the computations of one or more stepswith greater speed than using general-purpose computers or processorcores. In one embodiment, the optimization and simulation processesdescribed in this disclosure may include iterative processes of revisingand recalculating possible solutions, so as to minimize either the totalnumber of shots, or the total charged particle beam writing time, orsome other parameter. In another embodiment, an initial set of shots maybe determined in a correct-by-construction method, so that no shotmodifications are required.

While the specification has been described in detail with respect tospecific embodiments, it will be appreciated that those skilled in theart, upon attaining an understanding of the foregoing, may readilyconceive of alterations to, variations of, and equivalents to theseembodiments. These and other modifications and variations to the presentmethods for fracturing, manufacturing a surface, and manufacturing anintegrated circuit may be practiced by those of ordinary skill in theart, without departing from the spirit and scope of the present subjectmatter, which is more particularly set forth in the appended claims.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tobe limiting. Steps can be added to, taken from or modified from thesteps in this specification without deviating from the scope of theinvention. In general, any flowcharts presented are only intended toindicate one possible sequence of basic operations to achieve afunction, and many variations are possible. Thus, it is intended thatthe present subject matter covers such modifications and variations ascome within the scope of the appended claims and their equivalents.

1. A method for fracturing or mask data preparation or proximity effectcorrection for charged particle beam lithography comprising: setting abase dosage level for each of a plurality of exposures passes, whereinthe base dosage levels for at least two of the exposure passes aredifferent; and determining a plurality of shots for each of theplurality of exposure passes.
 2. The method of claim 1 wherein shots inthe plurality of shots for each of the plurality of exposure passes haveunassigned shot dosages.
 3. The method of claim 1 wherein shots in theplurality of shots for each of the plurality of exposure passes haveassigned shot dosages.
 4. The method of claim 3 wherein for each of theplurality of exposure passes, the assigned shot dosages are expressed asfractional multiples of the base dosage.
 5. The method of claim 3wherein the assigned shot dosages may have one of less than 17 dosagelevels.
 6. The method of claim 1 wherein the sum of the base dosagelevels for all the exposure passes equals a normal dosage.
 7. The methodof claim 1 wherein shot outlines for shots in a subset of the pluralityof shots in at least one of the plurality of exposure passes overlapeach other.
 8. The method of claim 1 wherein for an exposure pass in theplurality of exposure passes, shots in the plurality of shots aredisjoint.
 9. The method of claim 1 wherein the step of determiningcomprises using charged particle beam simulation.
 10. The method ofclaim 9 wherein the charged particle beam simulation includes at leastone of the group consisting of forward scattering, backward scattering,resist diffusion, coulomb effect, etching, fogging, loading and resistcharging.
 11. A method for forming a set of patterns on a surface usingcharged particle beam lithography comprising: a) setting a base dosagelevel for an exposure pass; b) exposing a plurality of shots for theexposure pass using the set base dosage level; and c) repeating steps a)and b) for at least one additional exposure pass, wherein the sum of thepluralities of shots for all of the exposure passes can form the set ofpatterns on the surface, and wherein the base dosage levels for at leasttwo of the exposure passes are different.
 12. The method of claim 11,further comprising using a charged particle beam writer that supportsonly unassigned shot dosage input shots.
 13. The method of claim 11wherein in the step of exposing, shots in the plurality of shots for theexposure pass have assigned shot dosages.
 14. The method of claim 13wherein the assigned shot dosages are expressed as fractional multiplesof the base dosage.
 15. The method of claim 13, further comprising usinga charged particle beam writer that supports as input data only shotshaving less than 17 shot dosage levels.
 16. The method of claim 11wherein the sum of the base dosage levels for all the exposure passesequals a normal dosage.
 17. The method of claim 11 wherein in the stepof exposing, shots in a subset of the plurality of shots overlap eachother, for at least one of the exposure passes.
 18. The method of claim11, further comprising using a charged particle beam writer thatsupports only disjoint shots within an exposure pass.
 19. The method ofclaim 11 wherein the surface is a semiconductor wafer, the methodfurther comprising using the set of patterns on the wafer to manufacturean integrated circuit.
 20. A method for manufacturing an integratedcircuit using an optical lithographic process, the optical lithographicprocess using a reticle, the method comprising: a) providing a chargedparticle beam source; b) setting a base dosage level for an exposurepass; c) exposing a plurality of shots for the exposure pass using theset base dosage level; and d) repeating steps b) and c) for at least oneadditional exposure pass, wherein the sum of the pluralities of shotsfor all of the exposure passes can form a set of desired patterns on thereticle, and wherein the base dosage levels for at least two of theexposure passes are different.
 21. The method of claim 20, furthercomprising using a charged particle beam writer that supports onlyunassigned shot dosage input shots.
 22. The method of claim 20 whereinthe sum of the base dosage levels for all the exposure passes equals anormal dosage.
 23. The method of claim 20, further comprising using acharged particle beam writer that supports only disjoint shots within anexposure pass.
 24. A system for fracturing or mask data preparation orproximity effect correction for use with shaped beam charged particlebeam lithography comprising: an input device capable of receiving a setof patterns to be formed on a surface; a computation device capable ofdetermining a set of shots that can be used to form the set of patternsusing a plurality of exposure passes, wherein each exposure pass has abase dosage level, and wherein the base dosage levels for at least twoof the exposure passes are different; and an output device capable ofoutputting the determined set of shots.
 25. The system of claim 24,wherein shots in the set of shots that can be determined by thecomputation device have unassigned shot dosages.